I try to solve Navier Stokes equation for 2-D flow in annulus (by using finnite difference method). I used simple algorithm and Alternating direction implicit method (Gauss Elemination) . Solution is converged, but after 5 million iterations for every time step. My grid is loose. If all (p primes) are smaller than ε, i accept solution is converged. What could be problem. Could it be not using finnite volume method or my converge criteria.